Embodiments of the invention relate generally to radio frequency (RF) communications systems, and more particularly to RF micro-electromechanical systems (MEMS) communications systems having a selectively increased characteristic impedance that reduces insertion losses, with the structure of the system also providing for improved yields during fabrication thereof.
RF MEMS devices are a technology that, in its most general form, can be defined as miniature devices that use an electrically actuated mechanical movement to achieve an open circuit or a closed circuit in a RF transmission line. When the RF MEMS device is in an on-position, the RF transmission line is “closed” and the RF MEMS device can be used to conduct a high-frequency RF signal. It is recognized that RF MEMS devices are ideal for providing such switching capability between open and closed circuits due to their desirable RF properties, including low radiative loss, low capacitive open state coupling (300 fFd), and very small mechanical geometry (76 um), resulting in minimal inductive parasitics and relatively low contact resistance (1 ohm).
One application of RF MEMS devices is for use in electronically steered antenna (ESA) systems, which are systems that combine the signals from multiple stationary antenna elements to point a beam of radio waves at a certain angle in space. The characteristics and angle of the beam may be controlled via an electronic steering of the beam in different directions without physically moving the antennas, with true time delay (TTD) being one known technique for doing so. Beam steering via TTD is accomplished by changing the path length or transmission time of each antenna element, which may be achieved by providing a TTD module that includes a plurality of RF MEMS devices coupled to RF transmission lines of various lengths. The amount of time it takes for a signal to be transmitted between the common feed point and the antenna is controlled by selecting a particular combination of transmission lines via switching of the RF MEMS devices, which imparts a desired amount of phase or time delay on the RF signal to each element.
It is recognized, however, that the use of RF MEMS devices and accompanying RF transmission lines for existing RF transmission systems (including ESA systems that utilize TTD) has numerous limitations and challenges associated therewith. One primary challenge is achieving a desired characteristic impedance of 50 Ohms in the system—which is the standard characteristic impedance utilized in most RF transmission systems. That is, due to the size of the RF MEMS devices and RF transmission lines in such systems, it is often difficult to achieve a characteristic impedance of 50 Ohms due to challenges associated with the miniaturization of the system. For example, characteristic impedance may be desirably altered by changing the width of the RF transmission lines or a spacing between the RF transmission lines, but such altering would result in increased resistance in the system (if the RF transmission lines are narrowed) or an increased size of the system (if spacing between the RF transmission lines is increased). As another example, characteristic impedance may be desirably altered by reducing a thickness of the insulating substrate (e.g., glass) upon which the RF transmission lines are formed in the system, but such thinning of the substrate may lead to poorer yields during fabrication due to the fragility of the substrate and potential breakage thereof that might occur with such reduced thickness.
Therefore, it would be desirable to provide an RF MEMS transmission system that provides a desirable characteristic impedance while addressing yield issues during fabrication. It would further be desirable to provide an RF MEMS transmission system with low RF insertion loss (<4 dB) that enables passive beamformer assemblies and maintains good signal transmission for broadband frequency signal processing applications.